Castable magnesium alloys

ABSTRACT

This invention relates to magnesium-based alloys particularly suitable for casting applications where good mechanical properties at room and at elevated temperatures are required. The alloys contain: 2 to 4.5% by weight of neodymium; 0.2 to 7.0 of at least one rare earth metal of atomic No. 62 to 71; up to 1.3% by weight of zinc; and 0.2 to 0.7% by weight of zirconium; optionally with one or more other minor component They are resistant to corrosion, show good age-hardening behaviour, and are also suitable for extrusion and wrought alloy applications.

This invention relates to magnesium-based alloys particularly suitablefor casting applications where good mechanical properties at room and atelevated temperatures are required.

Because of their strength and lightness magnesium-based alloys arefrequently used in aerospace applications where components such ashelicopter gearboxes and jet engine components are suitably formed bysand casting. Over the last twenty years development of such aerospacealloys has taken place in order to seek to obtain in such alloys thecombination of good corrosion resistance without loss of strength atelevated temperatures, such as up to 200° C.

A particular area of investigation has been magnesium-based alloys whichcontain one or more rare earth (RE) elements. For example WO 96/24701describes magnesium alloys particularly suitable for high pressure diecasting which contain 2 to 5% by weight of a rare earth metal incombination with 0.1 to 2% by weight of zinc. In that specification“rare earth” is defined as any element or mixture of elements withatomic Nos. 57 to 71 (lanthanum to lutetium). Whilst lanthanum isstrictly speaking not a rare earth element it is intended to be covered,but elements such as yttrium (atomic No 39) are considered to be outsidethe scope of the described alloys. In the described alloys optionalcomponents such as zirconium can be included, but there is norecognition in that specification of any significant variation in theperformance in the alloys by the use of any particular combination ofrare earth metals.

WO 96/24701 has been recognised as a selection invention over thedisclosure of a speculative earlier patent, GB-A-66819, which teachesthat the use of 0.5% to 6% by weight of rare earth metals of which atleast 50% consists of samarium will improve the creep resistance ofmagnesium base alloys. There is no teaching about castability.

Similarly in U.S. Pat. No. 3,092,492 and EP-A-1329530 combinations ofrare earth metals with zinc and zirconium in a magnesium alloy aredescribed, but without recognition of the superiority of any particularselection of any combination of rare earth metals.

Among commercially successful magnesium-rare earth alloys there is theproduct known as “WE43” of Magnesium Elektron which contains 2.2% byweight of neodymium and 1% by weight of heavy rare earths is used incombination 0.6% by weight of zirconium and 4% by weight of yttrium.Although this commercial alloy is very suitable for aerospaceapplications, the castability of this alloy is affected by its tendencyto oxidize in the molten state and to show poor thermal conductivitycharacteristics. As a result of these deficiencies special metalhandling techniques may have to be used which can not only increase theproduction costs but also restrict the possible applications of thisalloy.

There is therefore a need to provide an alloy suitable for aerospaceapplications which possesses improved castability over WE43, whilstmaintaining good mechanical properties.

SU-1360223 describes a broad range of magnesium-based alloys whichcontains neodymium, zinc, zirconium, manganese and yttrium, but requiresat least 0.5% yttrium. The specific example uses 3% yttrium. Thepresence of significant levels of yttrium tends to lead to poorcastability due to oxidation.

In accordance with the present invention there is provided a magnesiumbased alloy having improved castability comprising:

-   -   at least 85% by weight of magnesium;    -   2 to 4.5% by weight of neodymium;    -   0.2 to 7.0% of at least one rare earth metal of atomic No. 62 to        71;    -   up to 1.3% by weight of zinc; and    -   0.2 to 1.0% by weight of zirconium;    -   optionally with one or more of:    -   up to 0.4% by weight of other rare earths;    -   up to 1% by weight of calcium;    -   up to 0.1% by weight of an oxidation inhibiting element other        than calcium;    -   up to 0.4% by weight of hafnium and/or titanium;    -   up to 0.5% by weight of manganese;    -   no more than 0.001% by weight of strontium;    -   no more than 0.05% by weight of silver;    -   no more than 0.1% by weight of aluminium;    -   no more than 0.01% by weight of iron; and    -   less than 0.5% by weight of yttrium;    -   with any remainder being incidental impurities.

In the alloy of the present invention it has been found that theneodymium provides the alloy with good mechanical properties by itsprecipitation during the normal heat treatment of the alloy. Neodymiumalso improves the castability of the alloy, especially when present inthe range of from 2.1 to 4% by weight. A particularly preferred alloy ofthe present invention contains 2.5 to 3.5% by weight, and morepreferably about 2.8% by weight of neodymium.

The rare earth component of the alloys of the present invention isselected from the heavy rare earths (HRE) of atomic numbers 62 to 71inclusive. In these alloys the HRE provides precipitation hardening, butthis is achievable with a level of HRE which is much lower thanexpected. A particularly preferred HRE is gadolinium, which in thepresent alloys has been found to be essentially interchangeable withdysprosium, although for an equivalent effect slightly higher amounts ofdysprosium are required as compared with gadolinium. A particularlypreferred alloy of the present invention contains 1.0 to 2.7% by weight,more preferably 1.0 to 2.0% by weight, especially about 1.5% by weightof gadolinium. The combination of the HRE and neodymium reduces thesolid solubility of the HRE in the magnesium matrix usefully to improvethe alloy's age hardening response.

For significantly improved strengthening and hardness of the alloy thetotal RE content, including HRE, should be greater than about 3% byweight. By using an HRE there is also a surprising improvement in thealloy's castability, particularly its improved microshrinkage behaviour.

Although the heavy rare earths behave similarly in the present alloys,their different solubilities result in preferences. For example,samarium does not offer the same advantage as gadolinium in terms ofcastability combined with good fracture (tensile) strength. This appearsto be so because if samarium were present in a significant amount excesssecond phase would be generated at grain boundaries, which may helpcastability in terms of feeding and reduced porosity, but would notdissolve into the grains during heat treatment (unlike the more solublegadolinium) and would therefore leave a potentially brittle network atthe grain boundaries, resulting in reduced fracture strength—see theresults shown in Table 1. TABLE 1 (Wt %) Y.S UTS Melt Identity Sm Zn NdGd Zr (Mpa) (Mpa) Elongation % Sm DF 8540/49 1.15 0.73 2.5 0 0.5 164 2181.5 containing (average of 2 melts) Alloys Gd DF 8548 0 0.77 2.5 1.5 0.5167 295 7 containing Alloys

The presence of zinc in the present alloys contributes to their good agehardening behaviour, and a particularly preferred amount of zinc is 0.2to 0.6% by weight, more preferably about 0.4% by weight. Furthermore bycontrolling the amount of zinc to be from 0.2 to 0.55% by weight withthe gadolinium content up to 1.75% by weight good corrosion performanceis also achievable.

Not only does the presence of zinc alter the age hardening response of amagnesium-neodymium alloy, but also zinc changes the alloy's corrosionbehaviour when in the presence of an HRE. The complete absence of zinccan lead to significantly increased corrosion. The minimum amount ofzinc needed will depend upon the particular composition of the alloy,but even at a level only just above that of an incidental impurity zincwill have some effect. Usually at least 0.05% by weight and more oftenat least 0.1% by weight of zinc is needed to obtain both corrosion andage-hardening benefits. Up to 1.3% by weight the onset of over-ageing isusefully delayed, but above this level zinc reduces the peak hardnessand tensile properties of the alloy.

In the present alloys zirconium functions as a potent grain refiner, anda particularly preferred amount of zirconium is 0.2 to 0.7% by weight,particularly 0.4 to 0.6% by weight, and more preferably about 0.55% byweight.

The function and the preferred amounts of the other components of thealloys of the present invention are as described in WO 96/24701.Preferably the remainder of the alloy is not greater than 0.3% byweight, more preferably not greater than 0.15% by weight.

As regards the age hardening performance of the alloys of the presentinvention, up to 4.5% by weight of neodymium can be used, but it hasbeen found that there is a reduction in tensile strength of the alloy ifmore than 3.5% by weight is used. Where high tensile strength isrequired, the present alloys contain 2 to 3.5% by weight of neodymium.

Whilst the use in magnesium alloys of a small amount of the mixture ofneodymium and praseodymium known as “didymium” in combination with zincand zirconium is known, for example 1.4% by weight in U.S. Pat. No.3,092,492, there is no recognition in the art that the use of 2 to 4.5%by weight of neodymium in combination with from 0.2 to 7.0%. preferablyfrom 1.0 to 2.7%, by weight of HRE gives rise to alloys which not onlyhave good mechanical strength and corrosion characteristics but whichalso possess good castability qualities. In particular, it has beenfound that by using a combination of neodymium with at least one HRE thetotal rare earth content of the magnesium alloy can be increased withoutdetriment to the mechanical properties of the resulting alloy. Inaddition, the alloy's hardness has been found to improve by additions ofHRE of at least 1% by weight, and a particularly preferred amount of HREis about 1.5% by weight. Gadolinium is the preferred HRE, either as thesole or major HRE component, and it has been found that its presence inan amount of at least 1.0% by weight allows the total RE content to beincreased without detriment to the alloy's tensile strength. Whilstincreasing the neodymium content improves strength and castability,beyond about 3.5% by weight fracture strength is reduced especiallyafter heat treatment. The presence of the HRE, however, allows thistrend to continue without detriment to the tensile strength of thealloy. Other rare earths such as cerium, lanthanum and praseodymium canalso be present up to a total of 0.4% by weight.

Whilst in the known commercial alloy WE43 the presence of a substantialpercentage of yttrium is considered necessary, it has been found that inthe alloys of the present invention yttrium need not be present, andtherefore at the present time the alloys of the present invention can beproduced at lower cost than WE43. It has, however, been found that asmall amount, usually less than 0.5% by weight, of yttrium can be addedto the alloys of the present invention without substantial detriment totheir performance.

As with the alloys of WO 96/24701, the good corrosion resistance of thealloys of the present invention is due to the avoidance both ofdetrimental trace elements, such as iron and nickel, and also of thecorrosion promoting major elements which are used in other known alloys,such as silver. Testing on a sand cast surface according to the industrystandard ASTM B117 salt fog test yielded a corrosion performance of <100Mpy (Mils penetration per year) for samples of the preferred alloys ofthe present invention, which is comparable with test results of <75 Mpyfor WE43.

For the preferred alloys of the present invention with approximately2.8% neodymium, the maximum impurity levels in weight per cent are: Iron0.005, Nickel 0.0018, Copper 0.015, Manganese 0.03, and Silver 0.05.

The total level of the incidental impurities should be no more than 0.3%by weight. The minimum magnesium content in the absence of the recitedoptional components is thus 86.2% by weight.

The present alloys are suitable for sand casting, investment casting andfor permanent mould casting, and also show good potential as alloys forhigh pressure die casting. The present alloys also show good performanceas extruded and wrought alloys.

The alloys of the present invention are generally heat treated aftercasting in order to improve their mechanical properties. The heattreatment conditions can however also influence the corrosionperformance of the alloys. Corrosion can be dependent upon whethermicroscopic segregation of any cathodic phases can be dissolved anddispersed during the heat treatment process. Heat treatment regimessuitable for the alloys of the present invention include: SolutionTreat⁽¹⁾ Hot Water Quench Solution Treat Hot Water Quench Age⁽²⁾Solution Treat Cool in still air Age Solution Treat Fan air cool Age⁽¹⁾8 Hours at 520° C.⁽²⁾16 Hours at 200° C.

It has been found that overall a slow cool after solution treatmentgenerated poorer corrosion resistance, than the faster water quench.

Examination of the microstructure revealed that coring within the grainsof slow cooled material was less evident than in quenched material andthat precipitation was coarser. This coarser precipitate was attackedpreferentially leading to a reduction in corrosion performance.

The use of a hot water, or polymer modified quenchant, after solutiontreatment is therefore the preferred heat treatment route andcontributes to the excellent corrosion performance of the alloys of thepresent invention.

When compared with the known commercial magnesium zirconium alloy RZ5(equivalent to ZE41) which contains 4% by weight zinc, 1% by weight REand 0.6% by weight zirconium, it was found that the preferred alloys ofthe present invention showed a much lower tendency to suffer fromoxide-related defects. Such reduced oxidation is normally associated inmagnesium alloys with the presence of beryllium or calcium. However, inthe tested alloys of the present invention neither beryllium nor calciumwere present. This suggests that the HRE component—here specificallygadolinium—was itself providing the oxidation-reducing effect.

The following Examples are illustrative of preferred embodiments of thepresent invention. In the accompanying drawings:

FIG. 1 is a diagrammatic representation of the effect of the meltchemistry of alloys of the present invention on radiographic defectsdetected in the produced castings,

FIG. 2 is a graph showing ageing curves for alloys of the presentinvention at 150° C.,

FIG. 3 is a graph showing ageing curves for alloys of the presentinvention at 200° C.,

FIG. 4 is a graph showing ageing curves for alloys of the presentinvention at 300° C.,

FIG. 5 is a micrograph showing an area of a cast alloy containing 1.5%gadolinium scanned by EPMA in its as-cast condition,

FIG. 6 is a graph showing the qualitative distribution of magnesium,neodymium and gadolinium along the line scan shown in FIG. 5,

FIG. 7 is a micrograph showing an area of a cast alloy containing 1.5%gadolinium scanned by EPMA in its T6 condition,

FIG. 8 is a graph showing the qualitative distribution of magnesium,neodymium and gadolinium along the line scan shown in FIG. 7,

FIG. 9 is a graph showing the variation of corrosion with increasingzinc content of alloys of the invention in their T6 temper after hotwater quenching,

FIG. 10 is a graph showing the variation of corrosion with increasinggadolinium content of alloys of the invention in their T6 temper afterhot water quenching, and

FIG. 11 is a graph showing the variation of corrosion with increasingzinc content of alloys of the invention in their T6 temper after aircooling.

1. EXAMPLES Corrosion Testing 1

An initial set of experiments was carried out to determine the generaleffect of the following upon the corrosion performance of the alloys ofthe present invention:

-   -   Alloy chemistry    -   Melting variables    -   Surface Preparation Treatments

Melts were carried out with different compositions and different castingtechniques. Samples from these melts were then corrosion tested inaccordance with ASTM B117 salt fog test. Weight losses were thendetermined and corrosion rates calculated.

All melts were within the composition range of Table 2 below unlessotherwise stated, the remainder being magnesium with only incidentalimpurities. TABLE 2 Element Nd Zn Gd Fe Zr Composition 2.65-2.85 0.7-0.80.25-0.35 <0.003 0.45-0.55

All corrosion coupons (sand-cast panels) were shot blasted using aluminagrit and then acid pickled. The acid pickle used was an aqueous solutioncontaining 15% HNO₃ with immersion on this solution for 90 seconds andthen 15 seconds in a fresh solution of the same composition. Allcorrosion cylinders were machined and subsequently abraded with glasspaper and pumice. Both types of test piece were degreased beforecorrosion testing.

The samples were placed in the salt fog test ASM B117 for seven days.Upon completion of the test, corrosion product was removed by immersingthe sample in hot chromic acid solution.

Summary of Initial Results and Preliminary Conclusions

1. Chemical Composition

a) Effect of Neodymium—See Table 3 TABLE 3 Composition Melt CouponsChange ID mcd mpy 2% Nd DF8544 0.9 70 4% Nd DF8545 0.98 76.25“mcd” stands for mg/cm²/day

The effect of neodymium is negligible, and showed no significant effecton the rate of corrosion.

b) Effect of Zinc—See Table 4 TABLE 4 Composition Melt Coupons Change IDmcd mpy 0.5% Zn DF8488 0.5 42   1% Zn DF8490 0.7 56 1.5% Zn DF8495 1.6126

An increase in zinc of up to 1% has little effect but higher levels upto 1.5% increases corrosion.

c) Effect of Gadolinium—See Table 5 TABLE 5 Composition CouponsCylinders Change Melt ID mcd mpy mcd mpy   0% Gd DF8510 1.1 86 0.5 390.3% Gd DF8536 DF8542 1.0 82 0.17 14   1% Gd¹ DF8397 — — 0.29 23 1.5%Gd² DF8539 DF8548 1.2 89 0.17 14   2% Gd DF8535 DF8547 1.6 127 0.31 25¹The neodymium content was raised to 3% from 2.7%²The neodymium was reduced from 2.7% to 2.5% in both melts.

The addition of gadolinium has no significant effect on the corrosion ofthe alloy up to 1.5%. The much reduced corrosion of the cylinders wasnoted.

d) Effect of Samarium—See Table 6 TABLE 6 Composition Coupons CylindersChange Melt ID mcd mpy mcd mpy   0% Gd 0% Sm DF8510 1.1 86 0.5 39 1.5%Gd 0% Sm² DF8539 1.2 89 0.17 14 DF8548   0% Gd 1.5% Sm² DF8540 1.2 910.3 24 DF8549

The addition of Samarium to the alloy with no Gadolinium gives no changein the corrosion resistance of the alloy.

The replacement of Gadolinium with Samarium gives no change in thecorrosion resistance of the alloy.

e) Effect of Zirconium—See Table 7 TABLE 7 Composition Coupons CylindersChange Melt ID mcd mpy mcd mpy   0% Zr DF8581 2.48 194 — — (No Zirmax)  0% Zr DF 8509 0.7 56 0.3  28.5 (Zirmax De-iron DF 8587 12.10 944 — —only) 0.5% Zr DF8536 1.0 82 0.17 14 (5% Zirmax) DF8542

Generally, a lack of Zirconium resulted in very poor corrosionperformance.

2. Melting Variables

a) Cycling Melt Temperature before pouring Metal—See Table 8 TABLE 8Coupons Cylinders Casting Technique Melt ID mcd mpy mcd mpy SettledPlate DF8543-1 1.17 91 — — (constant temperature) Raised plate DF8501-10.4 32 0.5 37 (Cycled temperature) DF8543-2 1.17 91 — —

A constant temperature prior to casting improves settling of particles(some of which may be detrimental to corrosion performance). This testshowed no benefit.

b) Argon Sparging—See Table 9 TABLE 9 Casting Zirconium CouponsTechnique Melt ID Content mcd mpy Unsparged DF8581-1 (25 Kg melt 0.002.48 194 Plate no Zx) DF8588-1 (60 Kg melt 0.51 0.98 77 5% Zx) DF8602-1(60 Kg melt 0.51 0.49 38 5% Zx) Sparged Plate DF8581-2³ (25 Kg melt 0.020.42 33 5% Zx) DF8588-2⁴ (60 Kg melt 0.45 0.98 77 5% Zx) DF8602-2 (60 Kgmelt 0.48 0.48 37 5% Zx)⁴Argon Sparged for 30 mins.⁵Argon Sparged for 15 mins.

Argon sparging can improve the cleanliness of molten magnesium.

This data shows improved corrosion performance from some of the melts,two of which had been sparged. Note that Zr content was reduced in somecases by the sparging process.

a)Effect of Crucible Size—see Table 10 TABLE 10 Casting CouponsTechnique Melt ID mcd mpy 25 Kg Pot DF8536 0.9 71 DF8542 60 Kg PotDF8588-1 1.1 87 DF8602-1 0.49 38

The effect of the melt size is not conclusive in the corrosion rate ofthe alloy.

3. Metal Treatments

a) Effect of immersion in Hydrofluoric acid solution (HF)—See Table 11TABLE 11 Coupons Treatment Melt ID mcd mpy Not HF treated DF8543 1.2 91HF treated 0.5 37

The HF treatment of the alloy does significantly improve the corrosionperformance of the alloy.

b) Effect of Chromating (Chrome—Manganese)—See Table 12 TABLE 12 CouponsTreatment Melt ID mcd mpy Not Chromated DF8543 1.2 91 Chromated 1.2 96

Chromate treatment did not improve corrosion performance.

c) Effect of HF Immersion and Subsequent Chromate Treatment—See Table 13TABLE 13 Coupons Treatment Melt ID mcd mpy No Treatment DF8543 1.2 91 HFdipped then 1.1 87 Chromated

Use of Chromate conversion coatings on the alloy destroys the protectiondeveloped by immersion in HF.

These preliminary results and tentative initial conclusions were refinedin the course of the further work described in the following Examples.

2.EXAMPLES Corrosion Testing 2

Five sand-cast samples ¼″ thick in the form known as “coupons” weretested. The compositions of these coupons are set out in Table 14, theremainder being magnesium and incidental impurities. (“TRE” stands forTotal Rare Earths) TABLE 14 Composition (wt %) Melt ID Zn Zr Nd Gd TREFe MT 218923 0.75 0.55 2.59 1.62 4.33 0.003 MT 218926 0.8 0.6 2.5 0.43.0 0.003 MT 218930 0.8 0.6 3.5 0.4 4.0 0.003 MT 218932 0.8 0.5 3.5 1.55.2 0.003 MT 218934 0.75 0.6 2.6 1.5 4.3 0.003

The coupons were radiographed, and microshrinkage was found to bepresent within the coupons.

All the coupons were heat treated for 8 hours at 520° C. (968° F.), hotwater quenched, followed by 16 hours at 200° C. (392° F.).

The samples were grit blasted and pickled in 15% nitric acid for 90seconds then in a fresh solution for 15 seconds. They were dried andevaluated for corrosion performance for 7 days, to ASTM B117, in a saltfog cabinet.

After 7 days the samples were rinsed in tap water to remove excesscorrosion product and cleaned in hot Chromium-(IV)-Oxide (10%) and hotair dried.

The corrosion performance of the coupons is set out in Table 15. TABLE15 Corrosion rate Corrosion rate Melt ID (mcd) (mpy) MT 218923 0.84 66MT 218926 0.75 59 MT 218930 0.81 63 MT 218932 0.87 68 MT 218934 0.88 69

3.EXAMPLES Casting Testing

Casting trials were carried out to assess microshrinkage as a functionof alloy chemistry.

A series of casting were produced and tested having the targetcompositions set out in Table 16, the remainder being magnesium andincidental impurities. TABLE 16 Nd Gd Zn Zr 2.6 1.6 0.75 0.55 2.6 0.40.75 0.55 3.5 0.4 0.75 0.55 3.5 1.6 0.75 0.55All values shown are weight percent.

Melts were carried out under standard fluxless melting conditions, asused for the commercial alloy known as ZE41. (4% by weight zinc, 1.3%RE, mainly cerium, and 0.6% zirconium). This included use of a loosefitting crucible lid and SF₆/C0₂ protective gas.

Melt details and charges are provided in Appendix 1.

The moulds were briefly (Approximately 30 seconds—2 minutes) purged withC02/SF6 prior to pouring. The metal stream was protected with C0₂/SF₆during pouring.

For consistency, metal temperature was the same and castings were pouredin the same order for each melt. Melt temperatures in the crucible andmould fill times were recorded (see Appendix 1).

One melt was repeated (MT8923), due to a sand blockage in the down sprueof one of the 925 castings.

The castings were heat-treated to the T6 condition (solution treated andaged).

The standard T6 treatment for the alloys of the present invention is:

-   -   8Hours at 960-970° F. (515-520° C.)—quench into hot water    -   16 Hours at 392° F. (200° C.)—cool in air

The following components had this standard T6 treatment: Melt MT 8923 -1 off 925 Test bars and corrosion panels. Melt MT 8926 - 1 off 925 Testbars and corrosion panels. Melt MT 8930 - 1 off 925 Test bars andcorrosion panels. Melt MT 8932 - 2 off 925 Test bars and corrosionpanels. Melt MT 8934 - CH47. Test bars and corrosion panels.

Some variations were made to the quench stage after solution treatment,to determine the effect of cooling rate on properties and residualstresses in real castings.

Details are provided below:

-   -   Melt MT 8930—1 off 925 & test bars    -   8 Hours at 960-970° F. (515-520° C.)—fan air cool (2 fans)    -   16 Hours at 392° F. (200° C.)—cool in air    -   Melt MT 8926—1 off 925 & test bars    -   Melt MT 8934—1 off 925 & test bars    -   8 Hours at 960-970° F. (515-520° C.)—air cool (no fans)    -   16 Hours at 392° F. (200° C.)—cool in air

Temperature profiles were logged and recorded by embedding thermocouplesinto the castings.

ASTM test bars were prepared and were tested using an Instron tensilemachine.

The castings were sand blasted and subsequently acid cleaned usingsulphuric acid, water rinse, acetic/nitric acid, water rinse,hydrofluoric acid and final water rinse.

It was found that the alloys of the present invention were easy toprocess and oxidation of the melt surface was light, with very littleburning observed even when disturbing the melt during puddlingoperations at 1460 ° F.

The melt samples had the compositions set out in Table 17, the remainderbeing magnesium and incidental impurities. TABLE 17 Melt No. Nd Gd Zn FeZr TRE (wt %) MT8923-F2 2.6 1.62 0.75 0.003 0.55 4.33 MT8926-R 2.54 0.40.82 0.003 0.65 3.03 MT8930-R 3.48 0.4 0.82 0.003 0.60 4.0 MT8932-F2 3.61.6 0.77 0.003 0.53 5.38 MT8934-F2 2.59 1.62 0.74 0.003 0.57 4.35“TRE” stands for the Total Rare Earth content

The castings were tested for their mechanical properties and grain size.

a) Tensile Properties from Cast to Shape ASTM Bars Standard HeatTreatment (HWQ)—See Table 18 TABLE 18 0.2% PS UTS Grain Size Melt No MPa(KSI) MPa (KSI) Elongation mm (”) MT8923 183 (26.5) 302 (43.8) 7 0.015(0.0006) MT8926 182 (26.4) 285 (41.3) 6½ 0.016 (0.0006) MT8930 180(26.1) 265 (38.4) 5 0.023 (0.0009) MT8932 185 (26.8) 277 (40.2) 4 0.018(0.0007) MT8934 185 (26.8) 298 (43.2) 6 0.022 (0.009)

Detailed observations recorded during the inspection of the castings aresummarised as follows:

b) Surface Defects

All castings showed good visual appearance, with the exception of onemisrun in melt MT8932 (High Nd/Gd content).

Dye penetrant inspection revealed some micro shrinkage (subsequentlyconfirmed by radiography). The castings were generally very clean, withvirtually no oxide related defects.

The castings can be broadly ranked into the following groups: MT 8932(high Gd, high Nd) Best (except for misrun) MT 8923/34 (high Gd) SimilarMT 8930 (high Nd) MT8926 (low Gd) Worst

c) Radiography

Main defect was microshrinkage.

It is difficult to provide a quantitative summary of the effect of meltchemistry on radiographic defects, due to variations between castingseven from the same melts. FIG. 1 however attempts to show this bydiagrammatically ranking the average ASTM E155 rating for microshrinkage from all of the radiographic shots of each casting.

The following conclusions were reached:

A. Metal Handling

The alloys of the present invention proved to be easy for the foundry tohandle.

Equipment and melting/alloying is comparable with ZE41 and much simplerthan WE43.

Oxidation characteristics are similar or even better than ZE41. This isa benefit when alloying and processing the melt. Mould preparation isalso simpler since gas purging can be carried out using standardpractice for ZE41 or AZ91 (9% by weight aluminium, 0.8% by weight zincand 0.2% manganese). There is no need to purge and seal the moulds withan Argon atmosphere as is required for WE43.

B. Casting Quality

Castings were largely free of oxide related defects; where present theycould be removed by light fettling. This standard of surface quality ismore difficult to achieve with WE43, requiring much more attention tomould preparation and potential for rework.

The main defect present was microshrinkage. The present alloys areconsidered to be more prone to microshrinkage than ZE41.

Whilst changes in the rigging system (use of chills and feeders) are themost effective way to resolve microshrinkage, modifications to the alloychemistry can help. This latter point was addressed in this castingtrial.

A true assessment can only be achieved by the production of manycastings, however from this work the following general trends wereobserved:

-   -   Microshrinkage is reduced when Nd and/or Gd content is increased    -   Higher Nd shows a small increase in the tendency for segregation        to develop    -   High alloy content (particularly of Nd) appears to make the        molten metal slow to fill the mould. This can lead to misrun        defects.

C. Mechanical Properties

Tensile properties are good.

Yield strength is very consistent between all melts tested indicating awide tolerance to melt chemistry.

High Nd levels (3.5%) had the effect of reducing ductility and fracturestrength. This would be expected to be as a consequence of greateramounts of insoluble Nd rich eutectic.

High Gd levels (1.6%) did not reduce fracture strength or ductility. Ifany trend is present, an improvement in fracture strength is associatedwith higher Gd content. APPENDIX 1 MELT DETAILS MT8923, MT8926, MT8930,MT8932, MT8934 Input Material Analysis Nd Gd Zn Weight % Nd Hardener 26½— — Gd Hardener — 21 — (DF8631) Sample Ingot SF3739  2.64 0.42 0.87SF3740  2.68 1.29 0.86 Scrap Material MT8145  2.8 0.27

For all of the melts their zirconium contents were full, ie 0.55% byweight. Melt MT8923 Nd Gd Zn Weight % Target Composition 2.6 1.7 0.8Charge 279 lbs Sample Ingot (SF3740)  8 lb 4 oz Gd Hardener (DF8631 21%Gd)  2 lb 6 oz Nd Hardener (26.5% Nd)  18 lbs Zirmax

Procedure

Clean 3001b crucible used

09.00—Ingot began melting

10.15—Analysis sample taken

10.30—1400° F.—Hardeners added

10.45—1450° F.—Mechanical stirrer used for 3 minutes

10.50—1465° F.—Clean off melt surface

10.52—Analysis sample taken

10.58—1496° F.—Die bar taken and start of settle period

11.30—1490° F.—Lift crucible to pour Pouring Temperature Fill TimeCasting (° F.) (S) Comments ASTM Bars 1460 — — 925 # 1 1448  90+ NoFill - Downsprue Blocked Corrosion 1428 25 Plate 925 # 2 1422 51Corrosion 1415 21 Plate Weld Plate 1411 —

Melt MT8926 Nd Gd Zn Weight % Target Composition 2.56 0.4 0.8 Charge 269lbs Sample Ingot (SF3739)  0 lbs Gd Hardener (DF8631)  2.1 lbs NdHardener (26.5% Nd) 17.4 lbs  Zirmax

Procedure

Clean 300 lb crucible used

09.00—Start melt

09.00—Analysis sample taken

10.30—1400° F.—Addition made

10.40—1440° F.—Melt surface cleaned

10.45—1458° F.—Melt stirred as MT8923

10.50—1457° F.

10.55—1468° F.—Analysis sample and die bar taken

11.12—1494° F.

11.28—1487° F.—Lift crucible to pour

NB—Only ½ ingot left after pouring castings—need more metal PouringTemperature Casting (° F.) Fill Time (S) Comments ASTM Bars 1460 — 925 #3 1448 45 Corrosion 1438 16 Plate 925 # 4 1433 41 Corrosion 1426 20Plate Weld Plate 1420 19

Melt MT8930 Nd Gd Zn Weight % Target Composition 3.5 0.4 0.8 Charge 273lbs  Sample Ingot (SF3739) 0.12 lbs   Gd Hardener (DF8631) 14 lbs NdHardener 18 lbs Zirmax

Procedure

Clean 3001b crucible used

09.00—Melt started

10.10—Part melted

11.00—1400° F.—Alloyed hardeners

11.20—1465° F.—Melt stirred as MT8923

11.30—Die bar and analysis sample taken

11.40—1503° F.

12.05—1489° F.—Lift crucible to pour Pouring Temperature Casting (° F.)Fill Time (S) Comments ASTM Bars 1460 — 925 # 6 1447 46 Corrosion 143716 Plate 925 # 5 1432 51 Corrosion 1424 18 Plate Weld Plate 1419 —

Melt MT8932 Nd Gd Zn Weight % Target Composition 3.5 1.6 0.8 Charge  120lbs Scrap (ex MT8923)  160 lbs Sample Ingot (SF3740)  6.5 lbs GdHardener (DF8631) 17.1 lbs Nd Hardener   15 lbs Zirmax

Procedure

Clean 3001b crucible used

06.30—Melt started

08.00—1370° F.—Holding

09.00—1375° F.—Alloy hardeners

09.25—1451° F.—Puddle as MT8923

09.33—1465° F.—Cast analysis sample

09.45—1495° F.—Settling. Burner input 10% flame

09.50—1489° F.—Settling. Burner input 20% flame *

10.00—1490° F.—Cast final analysis block

-   -   Lift crucible

* Settle not quite as good as some melts—needed to increase burner nearend of melt Pouring Temperature Casting (° F.) Fill Time (S) CommentsASTM Bars 1460 — — 925 # 9 1452 60 RH riser (D Sprue furthest away) didnot fill all the way Corrosion 1438 19 Plate 925 # 7 1433 48 Corrosion1424 16 Plate Weld Plate 1420 16

Melt MT8934 Nd Gd Zn Weight % Target Composition 2.6 1.7 0.8 Charge  170lbs Scrap (ex MT8145)  113 lbs Sample Ingot (SF3740) 18.3 lbs GdHardener (DF8631)  2.9 lbs Nd Hardener 16.3 lbs Zirmax

Procedure

10.30—Melt charged into well cleaned crucible from previous melt

11.30—Melt molten and holding

12.05—1400° F.—Analysis block taken

-   -   —1402° F.—Hardeners alloyed

12.40—1430° F.

12.50—1449° F.—1461° F.—Melt puddle as MT8923

13.00—1461° F.—Analysis sample taken

13.05—1498° F.—Start settle

13.15—1506° F.

13.30—1492° F.—Burner input 17%

13.32—1491° F.—Lift crucible to pour Pouring Temperature Casting (° F.)Fill Time (S) Comments CH47 1450 35 (ZE41 is 31S) 925 # 8 1442 42 ASTMBars — — Corrosion — — Crucible virtually empty. Plate Metal qualitylikely to be poor in last moulds

4.EXAMPLES Ageing Trials

The hardness of samples of the preferred alloy of the present inventionwere tested and the results are set out in FIGS. 2 to 4 as a function ofageing time at 150, 200 & 300° C. respectively.

There is a general trend that the addition of gadolinium shows animprovement in the hardness of the alloy.

In FIG. 2 the alloy with the highest gadolinium content has consistentlybetter hardness. The hardness improvement over that after solutiontreating is similar for the alloys. Also the scope of the testing wasnot long enough for peak hardness to be achieved as hardening is shownto occur at a relatively slow rate at 150° C. As peak age has not beenreached, the effect of gadolinium on over-ageing at this temperaturecould not be investigated.

FIG. 3 still shows an improvement in hardness by gadolinium addition, aseven when errors are considered the 1.5% gadolinium alloy still hassuperior hardness throughout ageing and shows an improvement in peakhardness of about 5 MPa. The gadolinium addition may also reduce theageing time needed to achieve peak hardness and improve the over-ageproperties. After 200 hours ageing at 200° C. the hardness of thegadolinium-free alloy shows significant reduction, while the alloy with1.5% gadolinium still shows hardness similar to the peak hardness of thegadolinium-free alloy.

The ageing curves at 300° C. show very rapid hardening by all thealloys, reaching peak hardness within 20 minutes of ageing. The trend ofimproved hardness with gadolinium is also shown at 300° C. and the peakstrength of the 1.5% gadolinium alloy is significantly higher (˜10Kgmm⁻² [MPa]) than that of the alloy with no gadolinium. A dramatic dropin hardness with over-ageing follows the rapid hardening to peak age.The loss of hardness is similar for all alloys from their peak agehardness. The gadolinium-containing alloys retain their superiorhardness even during significant over-ageing.

FIG. 5 and FIG. 7 are micrographs showing the area through whichline-scans were taken on the ‘as cast’ and peak aged (T6) specimenrespectively. The probe operated at 15 kV and 40 nA. The two micrographsshow similar grain sizes in the two structures.

The second phase in FIG. 5 has a lamellar eutectic structure. FIG. 7shows that after T6 heat treatment there is still significant retainedsecond phase present. This retained second phase is no longer lamellarbut has a single phase with a nodular structure. Within the grains ofthe as-cast structure a large amount of coarse, undissolved particlesare also seen. These are no longer present in the heat-treated samples,which show a more homogeneous grain structure. The superimposed lines onthe micrographs show the placement of the 80 μm line scans.

FIG. 6 and FIG. 8 are plots of the data produced by the EPMA line scansfor magnesium, neodymium and gadolinium. They show qualitatively thedistribution of each element in the microstructure along the line scan.

The y-axis of each graph represents the number of counts relative to theconcentration of the element at that point along the scan. The valuesused are raw data points from the characteristic X-rays given from eachelement

The x-axis shows the displacement along the scan, in microns.

No standards were used to calibrate the counts to give actualconcentrations for the elements so the data can only give qualitativeinformation regarding the distribution of each element. The relativeconcentration of each element at a point cannot be commented on.

FIG. 6 shows that, as in the ‘as-cast’0 structure, the gadolinium andneodymium are both concentrated at the grain boundaries as expected fromthe micrographs, as the main peaks for both lie at approximately 7, 40 &80 microns along the scan. It also shows that the rare earth levels arenot constant within the grains as their lines are not smooth in betweenpeaks. This suggests that the particle seen in the micrograph (FIG. 5)within the grains may indeed contain gadolinium and neodymium.

There is also a dip in the line for magnesium at about 20 microns; thiscorrelates to a feature in the micrograph. This dip is not associatedwith an increase in neodymium or gadolinium, and therefore the featuremust be associated with some other element, possibly zinc, zirconium orsimply an impurity.

FIG. 8 shows the distribution of the elements in the structure of thealloy after solution treatment and peak ageing. The peaks in the rareearths are still in similar positions and still match the areas ofsecond phase at grain boundaries (˜5, 45 & 75 microns). The areasbetween the peaks have however become smoother than in FIG. 6, whichcorrelates to the lack of intergranular precipitates seen in FIG. 7. Thestructure has been homogenised by the heat treatment and theprecipitates present within the grains in the as-cast have dissolvedinto the primary magnesium phase grains.

The amount of second phase retained after heat treatment shows that thetime at solution treatment temperature may not be sufficient to dissolveall the second phase and a longer solution treatment temperature may berequired. However it may also be possible that composition of the alloyis such that it is in a two-phase region of its phase diagram. This isnot expected from the phase diagrams of Mg—Gd and Mg—Nd [NAYEB-HASHEMI1988] binary systems, however as this system is not a binary systemthese diagrams cannot be used to accurately judge the position of thesolidus line for the alloy. Therefore the alloy may have alloyingadditions in it that surpass its solid solubility, even at the solutiontreatment temperature. This would result in retained second phaseregardless of the length of solution treatment.

5. EXAMPLES Effect of Zinc, Gadolinium and Heat Treatment on theCorrosion Behaviour of the Alloys

The effect of varying composition and heat treatment regimes on thecorrosion behaviour of the alloys of the present invention wasinvestigated in detail. For comparison equivalent alloys without zincwere also tested.

For this series of tests samples of alloys in the form of sand-castplates of dimension 200×200×25 mm (8×8×1″) were cast from alloy melts inwhich the gadolinium and zinc levels were varied (see Table 19). Theneodymium and zirconium levels were kept within a fixed range asfollows:

-   -   Nd: 2.55-2.95 % by weight    -   Zr: 0.4-0.6% by weight

Samples from the edge and from the centre of each plate were subjectedto one of the following heat treatment regimes:

-   (i) Solution treatment followed by hot water quench (T4 HWA)-   (ii) Solution treatment followed by hot water quench and age (T6    HWA)-   (iii) Solution treatment followed by air cool* and age T6 AC)-   (iv) Solution treatment followed by fan cool and age (T6 FC)    * The rate of cooling for each sample during an air cool was 2°    C./s.

All solution treatments were conducted at 520° C. (968 F) for 8 hrs andageing was conducted at 200° C. (392 F) for 16 hrs.

The samples were alumina-blasted using clean shot to remove surfaceimpurities prior to acid pickling. Each sample was pickled (cleaned) in15% HN0₃ solution for 45 s prior to corrosion testing. Approximately0.15-0.3 m (0.006-0.012″) thickness of metal was removed from eachsurface during this process. The freshly pickled samples were subjectedto a salt-fog spray test (ASTMB117) for corrosion behaviour evaluation.The cast surfaces of the samples were exposed to the salt fog.

The corrosion test results are shown in FIGS. 9 to 11.

In the alloy samples of the invention which contained zinc, corrosionwas observed to occur predominantly in regions of precipitates whereasin equivalent very low zinc and zinc-free alloys corrosion occurredpreferentially at grain boundaries and occasionally at someprecipitates. The zinc content of the samples tested significantlyaffected corrosion behaviour; corrosion rates increased with increasingzinc levels. Corrosion rates also increased when the zinc content wasreduced to near impurity levels. Gadolinium contents also affectedcorrosion behaviour, but to a lesser extent that zinc content. Generallyin the T6 (HWQ) condition, alloys containing <0.65-1.55% gadolinium gavecorrosion rates <100 mpy providing that the zinc content did not exceed0.58%, whereas, alloys containing 1.55-1.88% gadolinium could generallycontain up to 0.5% zinc before corrosion rate exceeded 100 mpy. Ingeneral, it was observed that the alloys that had been hot waterquenched after solution treatment achieved lower corrosion rates thanalloys that had been air- or fan-cooled. This might possibly be due tovariations in distribution of precipitate between fast and slow cooledsamples.

6. EXAMPLES Gadolinium Limitations

Some experiments were carried out to investigate the effect of varyingthe amount of gadolinium as compared with replacing it with anothercommonly used RE, namely cerium. The results are as follows: AnalysisSample Nd Ce Gd Zn Zr (wt %) DF8794 3.1 1.2 — 0.52 0.51 DF8798 2.8 —1.36 0.42 0.52 DF8793 2.4 — 6 0.43 0.43 MT8923 2.6 — 1.62 0.75 0.55

Tensile Properties 0.2% YS UTS Sample (MPa) (MPa) Elongation (%) DF8794165 195 1 DF8798 170 277 5 DF8793 198 304 2 MT8923 183 302 7

All alloy samples were solution treated and aged prior to testing.

Comparison of samples DF8794 and DF8798 shows that when the commonlyused RE cerium is used in place of the HRE preferred in this invention,namely gadolinium, tensile strength and ductility are dramaticallyreduced.

A comparison of DF8793 and MT8923 shows that increasing the gadoliniumcontent to a very high level does not offer a significant improvement inproperties. In addition, the cost and increasing density (the density ofgadolinium is 7.89 compared with 1.74 for magnesium) militates againstthe use of a gadolinium content greater than 7% by weight. TABLE 19

7. EXAMPLES Wrought Alloy—Mechanical Properties

Samples were taken from a 19 mm (0.75″) diameter bar extruded from a 76mm (3″) diameter water-cooled billet of the following composition inweight percent, the remainder being magnesium and incidental impurities:% Zn 0.81 % Nd 2.94 % Gd 0.29 % Zr 0.42 % TRE 3.36

As with other test alloys where there is a difference between the TRE(Total Rare Earth content) and the total of the neodymium and HRE—heregadolinium—this is due to the presence of other associated rare earthssuch as cerium.

The mechanical properties of the tested alloy in its T6 heat treatmentcondition are shown in Table 20. TABLE 20 Tensile Properties 0.2% TestProof Tensile Temperature Heat Stress Stress Elongation Vickers (° C.)Treatment (MPa) (MPa) (%) Hardness 20 T6 134 278 22 75 250 T6 117 17330.0 —

1. A castable magnesium based alloy comprising: at least 85% by weightof magnesium; 2 to 4.5% by weight of neodymium; 0.2 to 7.0% of at leastone rare earth metal of atomic No. 62 to 71; up to 1.3% by weight ofzinc; and 0.2 to 1.0% by weight of zirconium; optionally with one ormore of: up to 0.4% by weight of other rare earths; up to 1% by weightof calcium; up to 0.1% by weight of an oxidation inhibiting elementother than calcium; up to 0.4% by weight of hafnium and/or titanium; upto 0.5% by weight of manganese; no more than 0.001% by weight ofstrontium; no more than 0.05% by weight of silver; no more than 0.1% byweight of aluminium; no more than 0.01% by weight of iron; and less than0.5% by weight of yttrium; with any remainder being incidentalimpurities.
 2. An alloy as claimed in claim 1 wherein the alloy contains2.5 to 3.5% by weight of neodymium.
 3. An alloy as claimed in claim 1wherein the alloy contains about 2.8% by weight of neodymium.
 4. Analloy as claimed in claim 1 wherein the alloy contains 1.0 to 2.7% byweight of gadolinium.
 5. An alloy as claimed in claim 1 wherein thealloy contains about 1.5% by weight of gadolinium.
 6. An alloy asclaimed in claim 1 containing at least 0.05% by weight of zinc.
 7. Analloy as claimed in claim 1 containing at least 0.1% by weight of zinc.8. An alloy as claimed in claim 1 wherein the alloy contains zinc in anamount of 0.2 to 0.6% by weight.
 9. An alloy as claimed in claim 1wherein the alloy contains zinc in an amount of about 0.4% by weight.10. An alloy as claimed in claim 1 wherein the alloy contains zirconiumin an amount of 0.4 to 0.6% by weight.
 11. An alloy as claimed in claim1 wherein the alloy contains zirconium in an amount of about 0.55% byweight.
 12. An alloy as claimed in claim 1 wherein the total rare earthcontent, including heavy rare earths, is greater than 3.0% by weight.13. An alloy as claimed in claim 1 wherein the alloy contains less than0.005% by weight of iron.
 14. An alloy as claimed in claim 1 which doesnot contain from 0.5 to 6% by weight of rare earth metals of which atleast 50% by weight consists of samarium, when zirconium is present inan amount of at least 0.4% by weight.
 15. A method of producing a castproduct including the step of sand casting, investment casting,permanent mould casting or high pressure die casting a magnesium basedalloy comprising: at least 85% by weight of magnesium; to 4.5% by weightof neodymium; 0.2 to 7.0% of at least one rare earth metal of atomic No.62 to 71; up to 1.3% by weight of zinc; and 0.2 to 1.0% by weight ofzirconium; optionally with one or more of: up to 1% by weight ofcalcium; up to 0.1% by weight of an oxidation inhibiting element otherthan calcium; up to 0.4% by weight of hafnium and/or titanium; up to0.5% by weight of manganese; no more than 0.001% by weight of strontium;no more than 0.05% by weight of silver; no more than 0.1% by weight ofaluminium; no more than 0.01% by weight of iron; and less than 0.5% byweight of yttrium; with any remainder being incidental impurities.
 16. Amethod as claimed in claim 15 including the step of age hardening thecast alloy at a temperature of at least 150° for at least 10 hours. 17.A method as claimed in claim 15 including the step of age hardening thecast alloy at a temperature of at least 200° C. for at least 1 hour. 18.A method as claimed in claim 15 including the step of age hardening thecast alloy at a temperature of at least 300° C.
 19. A method as claimedin claim 15 wherein the alloy does not contain from 0.5 to 6% by weightof rare earth metals of which at least 50% by weight consists ofsamarium, when zirconium is present in an amount of at least 0.4% byweight.
 20. A method as claimed in claim 15 including the steps ofsolution heat treating and then quenching the cast alloy.
 21. A methodas claimed in claim 20 wherein the quenching step is effected by hotwater or a hot polymer-modified quenchant.
 22. A cast product producedby a method as claimed in claim
 15. 23. A cast product produced by amethod as claimed in claim 15 when in its T6 temper.
 24. An extruded orwrought product when formed from an alloy as claimed in claim 1.